The BBC Trust chairman, Sir Michael Lyons, has decided against seeking a second term and is to step down in May next year.

Lyons has today written to the culture secretary, Jeremy Hunt, saying he does not want to be reappointed when his current four-year term ends.

The former local government executive was the first chairman of the BBC Trust, which replaced the board of governors as the corporation's governance and regulatory body in 2007 in the wake of the Hutton inquiry.

In his letter to Hunt, he defended the record of the trust, which has come in for criticism for being too much of a "cheerleader" for the BBC.

"For all the continuing debate, I am clear that this model is robust, workable and effective. I am proud of what we have achieved in safeguarding the BBC's independence against significant challenge, and bringing the interests of audiences in all their diversity to the centre of the BBC's thinking," Lyons wrote. "We have taken openness and transparency to a new level."

That has been a key area of conflict between Hunt and the BBC, however, with the culture secretary putting pressure on the corporation to publish more details about the amount it pays its stars – including individual salaries – and senior executives, and management resisting that demand.

In the past year the BBC Trust has helped push the director general, Mark Thompson, and his senior executives into more transparency over their pay and expenses and also to take pay cuts.

However, BBC sources said he had thought again about his role over the summer and decided it was too time-consuming.

"I have taken time over the summer to reflect on whether I would want to be considered for reappointment," Lyons wrote to Hunt.

"For all the positives associated with this agenda, I have to acknowledge that the role of chairman has been far more demanding than the nominal three to four days a week in the job specification," he said. "This workload has now reached a point where I am increasingly concerned that it is crowding out other appointments to which I remain committed and other activity that I wish to undertake. So balancing all the factors I have on reflection concluded that my preference would be to limit my appointment to a single term and not seek reappointment from next May."

Hunt said in a statement: "I'd like to thank Sir Michael Lyons for the all work he has done during his term as chair of the BBC Trust and wish him the best of luck for the future. I am grateful that he has let us know of his intentions in good time so we can begin the process of finding a replacement as soon as possible."

The shadow culture secretary, Ben Bradshaw, said: "The government must not use Michael Lyons's departure to further

undermine the BBC or tear up its charter. The new chair must be someone who is able and willing to defend the BBC and its independence and impartiality robustly."

There has been intense speculation about Lyons's role since the coalition government was formed.

Hunt has said in the past he would scrap the BBC Trust. He decided to retain it once elected but piled pressure on Lyons to force BBC management to comply with a string of requests, including providing more detail on talent pay and allowing the National Audit Office greater access to the BBC's accounts.

Lyons carried out a review into civil service jobs in 2004 for Gordon Brown when Brown was chancellor. Hunt pointed out to MPs earlier this year that Lyons is also "a former Labour councillor".

The government will begin a search for Lyons's successor in due course.

YouGov chairman Roger Parry is thought to be one of the leading contenders to become BBC chairman, along with existing trustee Dame Patricia Hodgson.

Parry advised Hunt on how to encourage commercially viable local television networks. Hodgson joined the BBC as a member of the founding team of the Open University and became its director of policy and planning before leaving to become chief executive of the Independent Television Commission.